1. Field of the Invention
The invention relates to an ammoniacal bath for the electrodeposition of palladium-silver alloys, containing a complex-bonded palladium, a silver compound and a mercapto compound.
2. Background Information
German Patent No. 1 221 874 relates to a method for the electrodeposition of pore-free palladium-silver coatings (thickness approximately 5 to 100 micrometers) from an ammoniacal palladium/nitrate and silver nitrate solution with a pH value of 7.5 to 11, at a temperature of 35.degree. to 90.degree. C., at a current density of 1 to 10 mA/cm.sup.2 and at a voltage of 0.5 to 7 volts. Good results are obtained if the solution in German Patent No. 1 221 874 contains 0.5 to 10 g of the metal(s) per liter of solution. However, it is also possible to employ solutions with up to 150 g and more of metal per liter.
Swiss Patent No. 649 582 describes a bath for electroplating of substrates with palladium or palladium alloys at a temperature of 20.degree. to 75.degree. C. and at a current density of 0.1 to 10 A/dm.sup.2. Swiss Patent No. 649 582 is distinguished in that it has a pH value of 6.5 to 9.5, buffered by ammonium/borate, amine/borate or alkaline metal borate, to prevent the undesirable precipitation of palladium or of the alloy metal (copper, cobalt, cadmium, gold, iron, indium, nickel, silver, tin, zinc) during the operation. The bath in Swiss Patent No. 649 582 contains 1 to 50 g/l of palladium in the form of a palladium-ammine complex (palladium(II)-di or-tetra-ammine complex), 0 to 20 g/l of the alloy metal, 10 to 50 g/l of borate and possibly up to 5 g/l of a brightener of an aromatic sulfonated imide or amide, aromatic alkaline metal sulfonate and/or aromatic sulfonic acid.
A similar bath is known from Platinum Metals Review, (1984), 28(3), 117 to 124. It contains palladium and silver in the form of ammine complexes Pd (NH.sub.3).sub.4 (NO.sub.3).sub.2 and Ag(NH.sub.3).sub.2 NO.sub.3 and has a pH value of 11.5.
EP 0 059 452 B1 and EP 0 073 236 B1 relate to methods for the electrodeposition of coatings of palladium and its alloys with silver, copper and/or nickel from baths containing complexes of palladium with aliphatic polyamines (1,3-diaminopropane, N,N,N',N'-tetramethylethylene diamine, 2-hydroxy-1,3-diaminopropane) as a palladium source and have a pH value between 7.5 and 13.5. A particular aqueous bath for electrodeposition of the palladium-silver alloys is composed of 69.6 g/l of silver(I)-oxide, 53.2 g/l of palladium(II)-chloride, 222 g/l 1,3-diaminopropane, 106.2 g/l of K.sub.3 PO.sub.4 and 86.5 g/l of K.sub.2 HPO.sub.4 and has a pH value of 11.3, set with KOH or H.sub.3 PO.sub.4. The bath temperature is between 40.degree. and 65.degree. C. and the current density is between 1.1 and 538 mA/cm.sup.2. The palladium-silver alloys are particularly suited as surfaces for electrical contacts.
German Patent Publication DE 39 35 664 C1 concerns an aqueous ammoniacal bath with a pH value above 8 for electrodeposition of palladium-silver alloys, in which palladium and silver are provided in the form of ammine complexes. Besides 5 to 50 g/l of palladium and 2 to 40 g/l of silver, the bath contains 1 to 50 g/l of an aliphatic or aromatic mercapto compound (mercapto acetic acid, mercapto propionic acid, mercapto succinic acid, thioglycerol, thiophenol, thiosalicylic acid) as brighteners and, if required, additionally the amide of an aliphatic carboxylic acid, by means of which the possible occurrence of black, silver-containing precipitates in the bath can be prevented to a great extent. Furthermore, borate and, as a conductive agent ammonium phosphate, ammonium acetate and/or ammonium nitrate, can be contained in the bath. Bright, ductile and pore- and crack-free coatings of palladium-silver alloys with up to 40 weight-% silver can be deposited from this bath at room temperature, and are particularly suited as contact layers for electrical contacts.